appears that you cannot lower the mounting to the beam web, as the brackets are already down and resting on the top of the lower flange, is that correct?
correct, There may be room to reposition holes in them though
Can you saw the vertical ends of the brackets, horizontally, half way up. Then lower the boards with the top part of the bracket still attached, overlapping the ends where it was sawed. Drill a couple holes through each overlap and bolt.
Thanks, I had considered cutting them off to lower it but your idea would save some work
Or, cut
off the top parts, keeping the vertical ends like above. bolt a 2x4 to the end plates so they run perpendicular out from the beam. Then screw the plywood onto the top of that.
Or, cut off the top horizontal part of the bracket completely, just at the bent top corner where the vertical ends change to horizontal. Then bend the verticals 90 into horizontal plane and bolt the wood on top of that?
Or, use a 2x4/2x6 bolted against the center web, where the bracket is now. Screw the edges of the wood boards onto the top edge of the 2x4 or 2x6 so they are just under the slider. Then make some sort of wood/steel brace from bottom of 2x4 to outer edge of plywoods. Maybe another sheet of plywood like the top.
Or, frame it like a wall. 2x4 against the beam as described above., plywood on top, and 2x4 stubs perpendicular to the beam. The 2x4 against the beam is like the top plate of a wall, the vertical studs nailed down from above in a wall become the horizontal supports out. The wall sheathing in a wall is your plywood shelf top surface.
Making out of wood is not a permanent long term solution, but the goal would be to experiment on where you want the shelves. once you know what works, make up a sketch of dimensions and I will weld you up a bracket for each side you can drill and bolt to the beam. You can add plywood onto the brackets.
k